1. Field of Invention
The present invention relates generally offshore drilling and production systems, which are employed, e.g., for drilling and producing subsea oil or gas wells. In particular, the invention relates to marine riser tensioners for maintaining a tensile force on a riser extending from a subsea wellhead assembly through an opening in a deck of a floating platform.
2. Description of Related Art
Offshore production platforms must support production risers from oil or gas wells that extend to the platform from subsea wells. This is accomplished through the use of riser tensioners or riser tensioning mechanisms that hold the riser in tension between the production platform and the wellhead. The riser tensioning mechanism maintains the riser in tension so that the entire weight of the riser is not transferred to the wellhead and to prevent the riser from collapsing under its own weight. The tensioning mechanism must therefore exert a continuous tensional force on the riser that is maintained within a narrow tolerance. Often, the production platform is a floating structure that moves laterally, vertically, and rotationally with respect to the fixed equipment at the seafloor. Thus, the riser tensioner mechanism must simultaneously provide support to a riser while accommodating the motion of the surface facility or platform.
Risers extend through a platform in a well slot, an opening in a deck of the platform for passage of the riser string. At a defined elevation within a platform's well slot, a riser's lateral motion is restricted by a guidance device that reacts laterally against a riser, preventing lateral displacement of the riser while still permitting vertical movement of riser in order to keep an upper termination of the riser within the boundaries Of the well slot. The portion of a riser's upper termination above and below the guidance device can still move laterally as the riser rotates about the location of the lateral guidance device. The magnitude of the lateral motion of the upper termination of the riser is directly proportional to its elevation above or below the guidance device. It is desirable to have the guidance device located proximate to equipment coupled to the upper termination of the riser to decrease movement of the portion above the guidance device. As a result, it may be desirable to place the guidance device on an upper portion of a riser tensioner frame of the riser tensioner system rather than on a lower platform deck where the tensioner system is mounted. This may create problems as the riser tensioner frame must be sufficiently strong to react to the lateral loading by the riser.
Riser tensioner system frames may comprise a multitude of components. In sonic prior art embodiments, the tensioner frame includes a tensioner frame ring formed of a multitude of straight elements welded together at angled joints. Legs extend from the deck into the well slot to mount to the tensioner frame ring. The legs will join the tensioner frame ring at coped joints. Generally, each component is welded together and, due to the angled and coped joints, this makes for difficult fabrication. In addition, the angles at each joint transfer the loading of the tensioner frame from the structural elements to the welds joining each element. Thus, the strength of the tensioner is placed on welds that may be located in positions and angles that are difficult to form. Improper welding may lead to a frame with a significantly reduced strength that is prone to early failure.